The present application represents an improvement in the method and apparatus for supplying gas mixtures to a diver's breathing apparatus as taught in British Pat. No. 1,342,155 (National Research Development Corporation) published Dec. 28th, 1973.
Self-contained underwater breathing apparatus can be grouped into three basic categories according to the configuration of the breathing circuit:
(i) open circuit PA1 (ii) semi-closed circuit PA1 (iii) closed circuit
A parameter of prime concern in the design and operation of such an apparatus is the partial pressure of oxygen delivered to the diver. This must be kept within limits over the depth range of the apparatus for all diver oxygen uptakes.
In open circuit apparatus, the diver breathes the supply gas mixture directly by drawing it through a demand regulator on inhalation and dumping it overboard through an exhaust valve on exhalation. The oxygen partial pressure of the gas being breathed is identical to that of the supplied gas and can be selected by choosing the right mixture.
In semi-closed and closed circuit apparatus, the diver breathes the gas circulating in a breathing loop which consists of a breathing bag or counterlung, a carbon dioxide scrubber and associated plumbing. Various gases are added to and removed from this loop, including the supply gas. Therefore, the diver is breathing a gas with an oxygen partial pressure which is the resultant of the various gas transfers going on and the oxygen partial pressure of this gas is usually not the same as that of the supply gas. In these two categories of apparatus, then, it is the breathing loop oxygen partial pressure which must be controlled; in particular, the oxygen partial pressure in the counterlung.
The most direct way of controlling the counterlung oxygen partial pressure is by closed loop feedback control using an oxygen sensor. This is the method used in closed circuit apparatus and results in the most accurate control and the least gas consumption. It is also the most expensive and complex.
The conventional method of controlling counterlung oxygen partial pressure in existing semi-closed circuit sets is an open-loop control method. It consists of controlling the supply gas mixture and mass flow rate to the breathing loop. The mixture is held constant by selection and the mass flow of mixture is held constant by a choked venturi (mass-flow jet). This gas supply configuration, coupled with the other gas transfers going on in the loop, results in a predictable counterlung oxygen partial pressure range. A disadvantage of this arrangement is that, if it is to do any better than open circuit apparatus as far as gas consumption is concerned, several mixtures and flow rates must be used to cover various depth ranges. This creates gas supply logistics problems and means that such an apparatus cannot be used from the surface down to its maximum depth on a given control setting.
The method of open loop control of counterlung oxygen partial pressure in semi-closed circuit apparatus as taught in the aforementioned British patent overcomes most of the above problems. However, it has been found that the traditional static analysis of counterlung oxygen partial pressure is not sufficient for the design of the gas control system for such apparatus as it is for conventional sets.